Noise figure measurement inconsistencies

[Quadsat]

Hello.
We see some inconsistencies in noise marker measurements when using 10Hz VBW and 100Hz RBW against other settings with the BB60C In general we have an increase in the noise value of about 1dB/Hz. Have you experienced something similar to this before and would you know the probable cause of this? i have attached some pictures of the results we are getting.

• This topic was modified 9 months, 2 weeks ago by Quadsat.
• This topic was modified 9 months, 2 weeks ago by Quadsat.
• This topic was modified 9 months, 2 weeks ago by Quadsat.

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[Andrew]

Hi,

You are seeing this because of the trace averaging you are performing. The trace averaging is being performed as dB, not in linear units, and thus will skew the results. You will want the “clear and write” trace type for noise floor measurement. To compensate for the noisier measurement without the additional averaging, you can increase the RBW/VBW ratio. Something like 100/1Hz would perform significant averaging. Increasing RBW would also have an averaging like effect (1000/1).

You will also want to leave the VideoUnits/Detector settings to Power/Average (as you already have them set).

Let me know if you have follow up questions,
Thanks

• This reply was modified 9 months, 1 week ago by Andrew.

Hello Andrew. what i wanted to highlight was that we were seeing a difference in results when creating a VBW skew and we are having a hard time verifying the results we are getting. the pictures i sent was to highlight a 1 dB difference after averaging. when using 100/10 skew. we are seeing a 2dB skew when running clear write measurement. Also what do you mean with the trace averaging skewing the final results due to being done in dB

I want to make sure that the trace “Type” is set to “Clear & Write” and not averaging when you make the noise marker measurement. Trace averaging will interfere with the noise marker measurement. The trace average control is located in the Trace control panel in the upper left of the Spike application.

With Clear & Write you should no longer be seeing a different between a 100/100 or 100/10 measurement. Can you verify this?

Can you also provide which version of Spike you are using?

Hello Andrew
I did some checks using the clear write function of the trace and recording the results.
I have then averaged the noise of each sweep and averaged again roughly 5 min worth of samples. I can see a clear correlation between having the RBW and VBW at the same setting VS the VBW being lower then the RBW. I have attached an excel document with the findings.

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Quadsat,
Can you please confirm that your trace type is Clear / Write? The log averaging when trace averaging is active will invalidate noise measurements.

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Hello Justin.
Yes i can confirm that the trace is Clear/write

[Justin Crooks]

The other thing that can invalidate noise measurements is having spur reject mode on. And, of course any averaging done in log units (e.g. averaging points as dBm instead of mW).
Can you confirm that when you “averaged” the sweeps it was in power units, not log units?

• This reply was modified 8 months, 2 weeks ago by Justin Crooks.

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